locking/selftest: Avoid false BUG report

The work-around for the expected failure is providing another failure :/

Only when CONFIG_PROVE_LOCKING=y do we increment unexpected_testcase_failures,
so only then do we need to decrement, otherwise we'll end up with a negative
number and that will again trigger a BUG (printout, not crash).

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Tested-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: d82fed75 ("locking/lockdep/selftests: Fix mixed read-write ABBA tests")
Signed-off-by: Ingo Molnar <mingo@kernel.org>